1. Field of the Invention
This invention relates to metals such as magnesium (Mg), cadmium (Cd), antimony (Sb), zinc (Zn) and tellurium (Te) that have purities of about 99.9999 wt % (6N) and above which have been obtained by heating feed metals and distilling them for purification. The invention also relates to a method and an apparatus for producing such high-purity metals.
2. Background Information
In the manufacture of semiconductor devices which are seeing increasing demand as the result of the recent sophistication of electronics and declining cost, the need to use feed metals of higher purity is ever increasing. The fabrication of semiconductor devices such as blue-light laser diodes presents a demand for high-purity magnesium. In particular, the development of double heterostructure blue-laser diode devices is highly dependent on the quality of the material used in the cladding layer. Metals of high purity such as high-purity magnesium (Mg) generally contain sulfur (S), sodium (Na), aluminum (Al), silicon (Si), potassium (K), calcium (Ca), chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni), copper (Cu), arsenic (As), antimony (Sb), lead (Pb), fluorine (F), phosphorus (P), chlorine (Cl), silver (Ag), bismuth (Bi), gallium (Ga), lithium (Li), molybdenum (Mo), titanium (Ti) and boron (B) (these elements contained in Mg are collectively referred to as impurities and the sum of their contents is referred to as the total impurity content; in the case where high-purity Mg is used as a semiconductor material, the inclusion of up to 100 ppm of zinc need not be particularly avoided and presents no problem in use; hence, a zinc content of up to 100 ppm is not dealt with as an impurity). The impurities in the high-purity magnesium used in the cladding layer of the double heterostructure blue-laser diode and for other purposes are by no means desirable for the performance of semiconductor lasers and this is another reason for the increasing need to produce magnesium and other metals of ultra-high purity. Magnesium and zinc are metals having comparatively high vapor pressures and more difficult to purify than other semiconductor materials by distillation.
In the conventional process of producing high-purity metals by purification through distillation of metals such as magnesium, the metal vapor generated by heating in a high-vacuum atmosphere is recovered by allowing it to solidify on cooling plates in the passageway of vapors. For example, International Patent Publication No. 502565/1999 describes a technique in which a plurality of baffle plates are provided over three zones in a passageway for the magnesium vapor generated by heating a magnesium feed within a crucible in a high-vacuum atmosphere and the magnesium vapor is cooled with the temperature of the baffle plates being controlled to decrease gradually toward the higher position, thereby utilizing the difference between the solidification temperatures of impurities in the magnesium vapor such that high-purity magnesium is fractionally solidified in a specified zone in the intermediate section.
However, it is difficult on an industrial scale to ensure that only the desired high-purity metal such as magnesium is efficiently cooled and recovered from the metal vapor in the passageway of vapors. If the separation of high-purity metal is to be achieved by the difference in solidification temperature, it is difficult to exclude the entrance of impurities having only a small difference in solidification temperature. In order to obtain the desired high-purity magnesium, the specified zone for recovery must maintain a very small temperature range but this only results in a very low yield. On the other hand, if one wants a higher yield, the purity of magnesium has to be lowered. If smaller cooling plates are used with a view to maintaining a smooth passage of vapors during recovery of the high-purity metal, the yield remain low and is within a limited range since the amount of recovery depends on the size of the cooling plates. If larger cooling plates are used, the vapor passageway becomes so narrow as to prevent the passage of metal vapors, again causing the yield to remain low in a limited range.